This invention pertains to a method and apparatus for heat sealing an electron gun mount, including a glass stem, into a neck of a cathode-ray tube and, in particular, a technique for removing the cullet during the heat-sealing process.
A standard cathode-ray tube (CRT) comprises a faceplate panel with a cathodoluminescent screen, a glass funnel having a protruding neck, and a mount containing an electron gun adapted to emit one or more beams of electrons for striking the screen. The mount includes a wafer-shaped glass stem on which the gun is mounted with lead wires for the gun electrodes projecting through the wafer. Round portions of glass called "fillets" surround the lead wires at the glass stem to provide a better seal around the lead wires. On the other side of the glass stem, opposite the gun, is an exhaust tubulation which extends away from the stem. The funnel typically is sealed to the faceplate panel in a high-temperature oven using a glass frit before the mount is sealed to the neck of the funnel. After the frit-sealing step, the mount is sealed to the glass neck by a heat-sealing apparatus which applies high-temperature flames to a localized area around the neck where the seal is to be formed.
During this mount-sealing process, the CRT is typically held in a vertical panel-up position by a cradle, and the mount is supported by a mount socket held in the top end of a spindle. The glass stem with the gun mounted thereon is upwardly inserted into the glass neck by the mount socket and spindle. The mount is aligned, so that the gun electrodes are aimed to properly strike the cathodoluminescent screen, and held thereat by the mount socket while heat from burners is applied to the outside of the neck proximate the stem. The burners are positioned around the vertical central axis of the CRT neck so that the neck softens, thins and then seals to the stem. Also, excess neck material that is lower than the stem is cut off and falls away from the neck as cullet. A two-tiered burner apparatus utilized for heat sealing an electron gun mount into a neck of a stationary cathode-ray tube is described in U.S. Pat. No. 4,589,902 issued to Colacello et al. on May 20, 1986.
In the process of sealing the mount into the neck of the CRT, the neck glass is melted and collapses against the glass stem on which the electron gun is mounted. Continued heating causes the molten neck glass to flow into the stem to form a seal, and gravity causes the excess neck glass, or cullet, to fall away from the seal, as it is cut off, and onto the mount socket where it is allowed to cool. The molten cullet contracts as it cools, and the molten edge solidifies and adheres to and around the mount socket. This cullet must either be cracked off the mount socket, which produces contaminating glass particles, or be reheated and then removed, as described in U.S. Pat. No. 4,165,227 issued to Nubani et al. on Aug. 21, 1979. Cracking the cullet off the socket produces glass debris which can enter the tube and produce blocked aperture scrap, or arcing in the electron gun region of the tube. Remelting the cullet requires extra heating and cooling processing steps. The present invention provides a method and apparatus for cullet removal which is significantly more effective than the above-mentioned techniques.